JPH02194146A - High strength hot rolled steel sheet having excellent cold workability and surface quality and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture the steel sheet having excellent cold workability and surface quality by subjecting a steel contg. specific ratios of C, Mn, S, Cu, P, Si, N, SolAl and B to hot rolling under specific conditions and coiling it. CONSTITUTION:A steel contg., by weight, 0.0005 to 0.015% C, 0.05 to 0.5% Mn, 0.001 to 0.030% S, 1.0 to 2.2% Cu, <=0.200% P, <=1.0% Si, <=0.0050% N, 0.002 to 0.10% SolAl, 0.0002 to 0.0030% B and the balance Fe with inevitable elements is subjected to hot rolling at the temp. of Ar3 or above. After that, the hot rolled steel band is coiled at <=500 deg.C, preferably at about <=350 deg.C. In this way, the generation of pearlite is evaded to obtain the high strength hot rolled steel sheet essentially consisting of a ferritic phase and having excellent cold workability and surface quality can be obtd.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same, which are used in fields of application that require extremely high formability and high product strength. .

[Conventional technology] Conventional hot-rolled high-strength steel sheets for processing have a carbon content of approximately 0.03% or more, and in addition to strengthening the structure by quenching using carbon, solid solution strengthening elements such as Mn, Si, and P are added. Ordinarily, it is manufactured by utilizing precipitation strengthening with carbonitrides such as Ti and Nb.

As the tensile strength of the high-strength steel plate obtained in this manner increases, the workability, especially the ductility thereof, decreases. Therefore, it is not possible to ensure high strength and high workability at the same time.

It is contradictory that high strength and workability must be ensured at the same time - B! ll! There is no technology that can adequately respond to this. One ideal technology is that when performing cold deformation, the strength of the steel sheet is low, and at the same time, the workability, especially ductility, is sufficiently high, and the strength of the processed product is high after the deformation is completed. As a product, it becomes a complex machined part and a strong part. An example of a technique based on this idea can be found in, for example, Japanese Patent Publication No. 17049/1983. In this case, the change from the solid solution state of Cu to its precipitated state is utilized. i.e. of strength.

Process it while it is still low, and then heat treat it to reduce Cu.
The aim is to increase the strength of processed parts by precipitating .

However, the technique of increasing the strength of a steel plate by precipitating solid-dissolved Cu by heat treatment and the heat treatment conditions, etc., as disclosed in Japanese Patent Publication No. 57-17049, have been well known for a long time. For example, A11oys of 1ronan
d copper (McGR Yasuga-he ILL
RO00 GOMPANY, Inc.

! 934).

[Problems to be Solved by the Invention] Recently, the degree of demand from users for the material properties of highly workable hot-rolled steel sheets is becoming higher and higher. In other words, the number of parts with complex shapes that require even higher processing deformation is increasing, and the need to reduce costs by minimizing the deformation process on the steel plate user side has increased in recent years. This is because Therefore, Moesho's special public service in Sho 57-170
The technical content described in Publication No. 49 does not meet the requirements of steel sheet users.

One of the recent strong demands from users for highly formable hot-rolled steel sheets is for the final product to have ultra-high strength. for example,
Conventionally, parts with a tensile strength of about 45 kgf/mm',
Recently, there has been a demand for manufacturing from steel plates with a strength of 60 kgf/me' or more. Therefore, it is necessary to develop a technology that simultaneously increases the strength of this steel sheet and provides high workability.

Next is the need for extremely high deformation performance when deforming steel plates. This means that the shapes of final parts are becoming more and more complex, and that it is necessary to provide steel plates that meet these demands. In addition, there is a strong desire to reduce the number of processing steps on the user side, and for this purpose, it is necessary to provide steel sheets with extremely high deformation processing performance.

Furthermore, there is a need for m+ elementization in the heat treatment process on the user side. Naturally, as a parts manufacturer aiming to reduce costs, it is necessary to be able to perform heat treatment in a short time and further increase productivity.

No conventional technology has been able to satisfactorily meet these recent demands for new steel sheets from steel sheet users. The present invention has developed a high-strength hot-rolled steel sheet with excellent cold workability and surface quality, and a method for manufacturing the same, to meet these demands.

[Means for Solving the Problems] First, the hot-rolled steel sheet for processing, which is the object of the present invention, will be explained.

The hot-rolled steel sheet for processing of the present invention has a C0.0 for the following reasons.
005~0.0+59b, Mn 0.05~0.5
N, S0.001~0.0:10! k, C
u 1.0-2.2%, P 0.200% or less,
Si 1.0'* or less, N 0.0050!4
Hereinafter, Sol, A ratio 0.002 to 0.10! 4.
B 0.0002 to 0.0030%, and other inevitable elements, and mainly consists of a single phase of ferrite avoiding the generation of pearlite, and if necessary, one or two of Ti and Nb are added. poles, and even Ni
Contain.

As a result of research on hot-rolled steel sheets in which various elements were added singly or in combination to Cu-added steel, the present inventors found that the amount of strength increase due to Cu precipitation changes with Cfl, and that by reducing Cu, conventionally known It was newly discovered that a much larger increase in strength can be obtained than the increase in strength due to the precipitation of Cu.

Figure 1 shows Mn 0.15! In, Si 0.02 turtle,
S 0.015tS, P0.014, N 0.0
020! In, Sol AfL0.03! In, Cu
1.34.

The basic component is steel containing 0.0001 B, and 0.000% Cl.
Steel with a change in the range of 0.015% to 0.0465% was melted, heated at 1050°C, hot rolled at ^3 points or more, bound to a plate thickness of 3.0in+, and rolled up at 300°C. It is a graph showing the relationship between Cu and tensile strength at 300°C.
Curve (b) shows the case where the hot rolled steel sheet was heat treated at 600°C for 10 minutes. The difference between curve (a) and curve (b) is the amount of increase in strength due to the precipitation of Cu. When Cl is 0.025% or more, the amount of increase in strength is about 15 kg/m112, while when C(it is 0. Below 01596, the strength increase is approximately 2
An extremely large increase in strength of 0 kg/l11112 can be obtained. Cu is 0.015! A large change in the tensile strength of the as-rolled hot rolled steel sheet is observed at I, but this strength difference cannot be explained only by the solid solution strengthening of C. Corresponding to this strength difference, a large change is observed in the elongation of the hot-rolled steel sheet as it is wound, starting from Cff1 or 0.0154k.

Figure 2 is the same as Figure 1. Tl; is a graph showing the relationship between elongation and Cl of a C++-containing hot rolled steel sheet. From the same figure, Cu
It is recognized that extremely high ductility can be ensured by controlling 1- to 0.0154 or less.

In this way, ctl is 0.01! The reason why the strength increase due to heat treatment is large when the ductility is high < rt is not yet clear when di or less, but if we dare to speculate, we can think of it as follows. That is, Cu segregates in steel, and the content of Cu is different between ferrite and pearlite, and is higher in pearlite. For this reason, Cu in pearlite has a higher degree of supersaturation relative to the equilibrium solid solubility than Cu in ferrite, and is in a state where it is more likely to precipitate. Therefore, even when wound at a low temperature of 300° C., if the Cu content is high and pearlite is present, some Cu will precipitate and become hard. On the other hand, C
When u is low and pearlite does not exist and there is a single ferrite phase, CCJ is dissolved in a supersaturated state and does not harden.

It is assumed that when these hot-rolled sheets are heat-treated at a high temperature of about 600''C, sufficient precipitation of Cu, which was in a supersaturated state, occurs, resulting in a further increase in strength.

In this way, it is necessary to reduce Cu as much as possible in order to ensure an extremely high increase in strength and extremely high ductility.

The lower limit of Cu is 0.0005%, which is the limit for industrial melting.
shall be. On the other hand, if Cu exceeds 0.015'4, the amount of increase in strength and ductility will decrease, and at the same time, there will be restrictions on the winding temperature of hot rolling when manufacturing a steel plate before processing. That is, this is because a hardened structure is generated and reduces the ductility of the steel sheet before processing. Therefore, Cu is 0.0005 to 0.0154
The range shall be .

Particularly preferable Cu is 0.000
5 to 0.00504.

On the other hand, in the aforementioned Special Publication No. 57-17049, C.
According to an actual example, u is 0.04!4, and the elongation of the as-hot-rolled steel sheet is 37.9%, and the tensile strength is 38. Ikg/mm2. On the other hand, 550℃
Strength due to heat treatment of llr, t rise m is 13.9 kg/m
It is a+2. This is C3 in the invention shown in the above publication.
In No. 31, contrary to the present invention, since a pearlite phase exists as a structure, some Cu has already precipitated even in the as-hot-rolled stage, and the ductility is significantly lower than that of the present invention, and the strength increase due to heat treatment is also significant. There are few.

The characteristic feature of the strength improvement after heat treatment in the present invention is that not only the overall strength increase due to heat treatment but also the local strength increase of the molded part due to local heating is large. Here, local heating refers to 1. For example, welding such as spot welding, arc welding, flash butt welding, and local heating f-stage (for example,
(high-energy beam (racer, electron beam) irradiation, plasma heating, high-frequency heating, burner heating, etc.)

FIG. 3 is a diagram showing the cross-sectional hardness distribution of the spot welded portion of the steel of the present invention. The figure shows that the steel of the present invention has a lower hardness in the nugget part due to a smaller amount of C than the comparative steel of the same strength, and that the hardness increases due to the precipitation of Cu in the heat affected zone.

FIG. 4 is a diagram showing the cross tensile strength of a spot weld of the steel of the present invention in comparison with that of a comparative steel.

From the figure, the cross tensile strength of the steel of the present invention is extremely high compared to that of the comparative steel, and when the nugget diameter, which is the appropriate welding current, is 5 (T
When comparing the cross tensile strength at a welding current of (t is the plate thickness), it is recognized that the strength is more than twice as strong. As shown in Figure 3, this is due to an increase in hardness due to the precipitation of Cu observed in the heat-affected zone.1 The steel of the present invention has a high local strength even with short-term heat input such as spot welding. It has the property of being able to rise.

FIG. 5 is a diagram showing the effect of the number of laser irradiations on the change in hardness of a steel plate when the steel of the present invention is irradiated with a laser. Laser irradiation conditions are CO2 gas laser, lo
kW, lOx 10 mm beam, irradiation time 0.05 seconds, irradiation interval 6 seconds. The hardness increases significantly after several laser irradiations.

In general, the risk of fracture in a strength member is often in a very limited area, and therefore it is rarely necessary to strengthen the whole part by heat treatment. Further, from the viewpoint of productivity and cost, it is desirable that the heat treatment of the molded product be carried out in a short period of time and continuously. Therefore, it is of great technical significance to strengthen only the parts at risk of fracture by short-term heat treatment.

One specific example is an automobile wheel disc. Wheels are one of the important safety parts, and their service life is controlled by the fatigue properties of the materials. Cracks occur in areas where there is large strain in the plate thickness direction such as the nut seat and the hat, decorative holes,
These are the edges of sheared holes such as bolt holes and spot welds between the disc and the rim, where fatigue strength is important.

FIG. 6 is a diagram showing the results of investigating the fatigue strength of the steel plate of the present invention before and after heat treatment (600° C. x 30 seconds).

Compared to comparative materials, the steel of the present invention has high fatigue strength, and especially after heat treatment, its tensile strength increases, resulting in extremely high fatigue strength. By applying localized heating to the points in the wheel where fatigue cracks are likely to occur, the life of the wheel can be significantly extended.

P is effective as an element to improve the strength and corrosion resistance of steel sheets, but when it is not necessary, the amount of P is 0.0:1.
% or less. On the other hand, when improving the strength and corrosion resistance of steel plates, 0.06 to 0.20! It is preferable to add P to . This is because, as will be described later, the addition of B improves the secondary processing cracking resistance, so the amount of P added can be increased compared to the case where B is not added, but still 0.
If it exceeds 20H, secondary processing cracks will occur in the steel plate, so this is set as the upper limit. Note that the addition of P is effective in improving the corrosion resistance of the steel sheet along with the addition of Cu.

Si is normally contained as an impurity in an amount of 0.03% or less, but as an element that increases the strength of the steel plate, it is contained in an amount of 1.0% or less, preferably 0.3 to t, o*7, depending on the required strength level.
Add 4. However, if it exceeds 1.0 t, scale will occur significantly during the hot rolling process and the surface quality of the tA plate will deteriorate, so the upper limit is set at 1.0%.

In order to improve the workability of the steel plate, it is preferable that the amounts of Mn and S be as low as possible, and the upper limit of each is set at 0.5%.

0.030%, preferably 0.05 to 0
．． 30! ! .

0.001~0.010! Totosu. If the Mn content is too low, surface flaws will easily occur on the steel plate, so the lower limit is set at 0.05! Let it be ei.

Lower Nfi is preferable to improve workability.
It shall be 0.005096 or less.

The Cup is kept in a solid solution state before processing, and the strength is increased by precipitating Cu by heat treatment after processing. Figure 7 shows the heat treatment time (heat treatment temperature 5
50°C) is a graph showing the amount of increase in strength (tensile strength after heat treatment - tensile strength as hot rolled) with Cu as a parameter; in the figure, curve (a) is Cu2. OH, curve (b)
is Cu 1.68! Curve (c) is CuL38*, curve (
d) is the case of Cu 0.7144. From the same figure, Cu
If it is less than 1.0%, the amount of increase in strength is insufficient as shown in curve (d). On the other hand, if the number exceeds 2.2, the surface quality will deteriorate, so Cu should be 1.0 to 2.2t, preferably 1.2 to 2.2t.
The range is 2.0t.

A is an element necessary for deoxidation, and Sol and Ai are 0.
002! If it is less than Ii, deoxidation will not be sufficient, while if it is too much, the amount of alumina produced will increase, which will have a negative impact on the surface quality of the steel, so the upper limit is set at 0.1096.

B is an important element in order to maintain good surface quality of the steel sheet of the present invention. In hot rolling of the steel of the present invention, the rolling end temperature must be 3 or higher in order to maintain good material quality of the steel plate.However, as mentioned above, in order to control solid solution or precipitation of Cu, It is necessary to keep C to 0.01!d or less, and (as C decreases, the Ar3 point of the steel increases, so it is necessary to raise the rolling end temperature. However, increasing the rolling end temperature , which causes an increase in the scale darkness generated during rolling, and this scale causes sand-like flaws to occur on the steel plate surface.Therefore, in order to obtain a steel plate with good material and surface quality, it is necessary to use Ar for low carbon steel. , it is necessary to add elements that lower the transformation point.

From this point of view, the present inventors investigated the influence of elements other than Ar3 in ultra-low carbon steel to which Cu was added, and found that the addition of B significantly lowers the Ar3 point.

Figure 8 is a diagram showing the effect of B on Ar3 of Ti-added ultra-low carbon steel containing 1.3!1IICu;
3, which corresponds to the cooling rate during hot rolling after heating to 000°C
These are the results of Ar 3-point measurement when cooling at a cooling rate of 0° C./S.

As can be seen from the figure, the Ar3 point decreases rapidly with the addition of B up to 0.0010t, and with further addition
The r3 point gradually decreases.

Figure 9 shows the increase in the transformation point of the steel shown in Figure 8 due to 5 processing steps.J,I! ! FIG. 3 is a diagram showing the occurrence of sand-like scale flaws in steel plates that have been rolled at a temperature of +30° C. for each steel. (By adding more than 1,0002! of B, the occurrence of scratches is prevented, so the lower limit is set to 0.
0002%. On the other hand, adding more than 0.0030 mm of B is disadvantageous in terms of cost. Note that addition of B in this range is also preferable in terms of improving secondary processing cracking resistance.

One or both of Ti and Nb were added at 0.0% each. Old ~ 0.2
When C and N are added in the range of 0.005 to 0.2 t at 96°, C and N are fixed, and the resulting steel sheet becomes a non-aging steel sheet. When the steel sheet becomes a non-aging steel sheet, there is no decrease in ductility due to aging, and a high ductility steel sheet with a negative layer is obtained.

Ti reacts with C, 0, N, S, etc. in steel, so it must be considered in conjunction with the amount of these elements, but in order to fix these elements and obtain a high degree of press workability, the amount is 0.01! On the other hand, adding more than 0.2* is disadvantageous in terms of cost.

Nb also reacts with C, 0, N, etc. in steel, so it must be considered in conjunction with these times, but by fixing these elements,
In order to obtain a high degree of press workability, it is necessary to add 0.0005% or more, while adding more than 0.2 t is disadvantageous in terms of cost.

Ni is effective in maintaining high quality surface quality of steel sheets and preventing hot embrittlement. 0.15-1.05 as necessary
1i may be added.

The hot embrittlement of Cu-added steel is caused by the Cu-enriched area formed under the scale generated on the steel surface becoming liquid when heated above its melting point and penetrating into the austenite grain boundaries. Therefore, in order to prevent hot embrittlement during the hot-rolling stage of the slab, it is ideal to heat the slab to a temperature below the melting point of the Cu-enriched portion, and preferably to a temperature below 1080°C.

However, since a decrease in heating temperature brings about an increase in rolling load, heating to 1080° C. or lower cannot necessarily be carried out depending on the performance of the rolling mill. In this case, adding Ni is effective. By adding Ni, Ni is also concentrated in the Cu-concentrated section, raising the melting point of the Cu-concentrated section. 0.15 of Ni
The effect is small if the addition amount is less than 1. Addition of Ni in excess of H is disadvantageous in terms of cost.

Addition of one or two of Ti and Nb mentioned above, Ni
Even if they are added singly, or two or more of them are added in combination, the effect is exerted.

Next, regarding the hot rolling process in the method for manufacturing the steel plate of the present invention, hot rolling is performed on a hot slab directly delivered from a continuous casting machine or a hot slab obtained by heating at a temperature of Ar3 or higher, Thereafter, it is wound up at a temperature of 500°C or less. If coiling is carried out at a temperature exceeding 500° C., precipitation of Cu will occur, and not only will a soft steel sheet with good workability not be obtained, but the amount of increase in strength due to heat treatment will be small. In the present invention, cl is limited to suppress Cu precipitation at the winding stage, and 5
By winding at a temperature of 00° C. or lower, most of the Cu is kept in a supersaturated solid solution state. However, if it is wound at a temperature exceeding 500°C, Cu will precipitate and become hard, so the first limit of the winding temperature is set at 500°C. In order to keep all of the Cu in a solid solution state, it is optimal to keep the winding temperature at 350° C. or lower. If the C content or Mn1l is high as in conventional steel, if it is rolled at a low temperature, a hard phase called martensite phase or bainite phase will be formed due to transformation and become hard, so to avoid this, a lower limit is set for the coiling temperature. must be established.

In the steel of the present invention, the C content and Mni are controlled to be low, and the hardenability is significantly suppressed, so there is no metallurgical lower limit temperature for the coiling temperature. However, if the winding temperature is lower than 100°C, the winding shape will become poor, resulting in deterioration of surface quality, so preferably the winding temperature is 350°C or lower and 100°C or higher.

On the other hand, according to the above-mentioned Japanese Patent Publication No. 57-17049, the winding temperature is limited to 350° C. or higher) 1 (450° C. or lower). At low temperatures below 350°C, phase transformation (
This is limited because processability deteriorates due to the occurrence of martensite or bainite transformation.

However, as mentioned above, since the present invention is limited to the extremely low carbon region, phase transformation does not occur even if the winding temperature is 350°C or lower, and therefore there is no problem in workability. Rather, low-temperature winding with a large amount of Cu in the solid solution 11 can be adopted.

The obtained hot-rolled sheet is subjected to heat treatment after forming to increase its strength, but from the viewpoint of heat treatment workability, it is extremely important to complete the heat treatment at as low a temperature as possible and in a short time. In the present invention, sufficient consideration has been given to this point, and the object is achieved with a short heat treatment.

For example, the purpose can be sufficiently achieved in a short time such that the heat treatment temperature is 750° C. or less and the heat treatment time is 30 minutes or less.

The steel plate of the present invention can be used, for example, in automobile frames, wheels 1,
Possible applications include reinforcing parts, pressure vessels, compressor covers, and bearings.

[Example] Next, the present invention will be specifically explained using an actual hh example.

Example 1 Steel slabs from A to M shown in Table 1 were heated and hot-rolled at the heating temperatures shown in the same table, and rolled up to a thickness of 4. A hot rolled steel sheet of On+m was obtained. Tensile test value of the steel plate and 1 at 600℃
Table 1 shows the increase in tensile strength ΔTS (tensile strength after heat treatment - tensile strength as hot rolled) due to heat treatment for 0 minutes and the presence or absence of sand-like scale defects. The steels A to H and M of the present invention also have excellent ductility, and their tensile strength increases markedly after a short heat treatment, and at the same time, there is no occurrence of sand-like scale. On the other hand, comparative steel (3) has good surface quality, but not only has a high C content and low ductility, but also has a small increase in tensile strength due to heat treatment. On the other hand, Comparative Steels J and J had excellent mechanical properties, but had several second scale defects.

Example 2 Steels Not and No2 having the compositions shown in Table 2 were hot rolled to obtain hot rolled steel plates having a thickness of 3.0 aus. These steel plates were formed into a pressure vessel. A sample was cut out from this pressure vessel. The thickness of the cut sample is approximately 26 tons.
Met. Tensile strength of this sample as is and 630
Table 3 shows the tensile strength after heat treatment at ℃ for 5 minutes (corresponding to stress relief annealing to remove the internal stress of the pressure vessel). The strength increase amount ΔTS in the same table is the value obtained by subtracting the tensile strength as hot rolled from the tensile strength after press forming and heat treatment.

While the comparative steel was significantly softened by the heat treatment after working, the steel of the present invention achieved a further increase in strength by the heat treatment after working.

Example 3 Steels No. 3 and No. 4 having the compositions shown in Table 4 were hot rolled to obtain hot rolled steel plates with a thickness of 2.0 m111. After pickling these steel plates, samples were cut out and spot welded.

Table 5 shows the spot welding conditions. In order to evaluate spot welds, we measured the shear tensile strength, upper chamber tensile strength, and nugget diameter at each welding current, and at the same time measured the cross-sectional hardness of the sample spot welded at a welding current that made the nugget diameter 5F. Distribution measurements were performed.

Table 5 Spot Welding Conditions Figure 3 shows the cross-sectional hardness distribution measurement results of the above samples, and it is observed that the hardness of the steel of the present invention corresponds to the precipitation of Cu at the back of the welding heat shadow. FIG. 4 shows the measurement results of cross tensile strength at each welding current. The steel of the present invention has high cross tensile strength even when the welding current is small, and when comparing the cross tensile strength at a current value that allows the nugget diameter to be 5 rK, which is an appropriate welding current value, the steel of the present invention has a high cross tensile strength. It has a strength that is more than twice that of steel. 10th
The figure shows the measurement results of shear tensile strength at each welding current. The steel of the present invention has higher shear tensile strength than the comparative steel at any welding current.

[Effects of the Invention] The present invention provides a novel heat treatment method that has extremely good cold workability and excellent surface quality, and allows the high strength required for the final product to be achieved by a short heat treatment after cold working. The present invention provides a rolled steel sheet and a new method for producing such a hot rolled steel sheet by simple means of regulating the composition and controlling the coiling temperature of the hot rolled steel sheet.
It fully meets new demands from steel sheet users, and has enormous industrial benefits.

[Brief explanation of the drawing]

Figure 1 is a graph showing the effect of C content on the strength of hot rolled steel sheets before and after heat treatment for Cu precipitation. Figure 2 is a graph showing the effect of C content on the ductility of hot rolled steel sheets. Figure 3 is a graph showing the cross-sectional accuracy distribution of the spot weld of the steel plate of the present invention, Figure 4 is a graph showing the influence of welding current on the cross tensile strength of the spot weld of the steel plate of the present invention, and Figure 5 is a graph showing the influence of welding current on the cross tensile strength of the spot weld of the steel plate of the present invention. Graph showing the change in hardness of the steel plate of the present invention depending on the number of laser irradiation passes. Figure 6 is a graph showing the fatigue properties of the steel plate of the present invention after heat treatment. Figure 7 is the increase in strength of the ultra-low carbon hot rolled steel plate. A graph showing the effect of heat treatment time on Cuil as a parameter. Figure 8 shows the effect of Ar and Bm on Cu-added ultra-low carbon steel
Figure 9 is a graph showing the effect of B content on the surface quality of steel sheets; Figure 1n is a graph showing the effect of welding current on the shear tensile strength of spot welds of the steel of the present invention. be.

Claims (1)

[Claims] 1. C0.0005-0.015% Mn 0.05-0.5% S 0.001-0.030% Cu 1.0-2.2%, P 0.200% or less Si1 .0% or lessN0.0050% or lessSol. Cold workability characterized by containing 0.002-0.10% Al, 0.0002-0.0030% B, the remainder consisting of Fe and other unavoidable elements, and mainly consisting of a single ferrite phase that avoids the generation of pearlite. and high-strength hot-rolled steel sheets with excellent surface quality. 2. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less Ni0.15 ~1.0% N0.0050% or less Sol. Cold workability characterized by containing 0.002-0.10% Al, 0.0002-0.0030% B, the remainder consisting of Fe and other unavoidable elements, and mainly consisting of a single ferrite phase that avoids the generation of pearlite. and high-strength hot-rolled steel sheets with excellent surface quality. 3. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less N0.0050 % or less Sol. In addition to Al0.002-0.10% B0.0002-0.0030%, one or both of Ti or Nb are contained in the range of 0.01-0.2% and 0.005-0.2%, respectively. A high-strength hot-rolled steel sheet with excellent cold workability and surface quality, which is characterized by being mainly composed of a single phase of ferrite, with the balance being Fe and other unavoidable elements, and avoiding the generation of pearlite. 4. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less Ni0.15 ~1.0% N0.0050% or less Sol. In addition to Al0.002-0.10% B0.0002-0.0030%, one or both of Ti or Nb are contained in the range of 0.01-0.2% and 0.005-0.2%, respectively. A high-strength hot-rolled steel sheet with excellent cold workability and surface quality, which is characterized by being mainly composed of a single phase of ferrite, with the balance being Fe and other unavoidable elements, and avoiding the generation of pearlite. 5. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less N0.0050 % or less Sol. A steel containing 0.002 to 0.10% Al and 0.0002 to 0.0030% B, with the balance consisting of Fe and unavoidable elements.
A method for producing a high-strength hot-rolled steel sheet with excellent cold workability and surface quality, characterized by hot rolling at a temperature of r_3 or higher and winding the obtained hot-rolled steel strip at a temperature of 500°C or lower. . 6. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less Ni0.15 ~1.0% N0.0050% or less Sol. A steel containing 0.002 to 0.10% Al and 0.0002 to 0.0030% B, with the balance consisting of Fe and unavoidable elements.
A method for producing a high-strength hot-rolled steel sheet with excellent cold workability and surface quality, characterized by hot rolling at a temperature of r_3 or higher and winding the obtained hot-rolled steel strip at a temperature of 500°C or lower. . 7. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less N0.0050 % or less Sol. In addition to Al0.002-0.10% B0.0002-0.0030%, it contains one or two types of Ti or Nb in the range of 0.01-0.2% and 0.005-0.2%, respectively. Then, the steel consisting of the balance Fe and unavoidable elements was heated to Ar_
The hot rolled steel strip obtained by hot rolling at a temperature of 3 or more is
A method for producing a high-strength hot-rolled steel sheet with extremely excellent cold workability, characterized by winding at a temperature of 00°C or lower. 8. By weight: C0.0005-0.015% Mn0.05-0.5% S0.001-0.030% Cu1.0-2.2% P0.200% or less Si1.0% or less Ni0.15 ~1.0% N0.0050% or less Sol. In addition to Al0.002-0.10% B0.0002-0.0030%, it contains one or two types of Ti or Nb in the range of 0.01-0.2% and 0.005-0.2%, respectively. Then, the steel consisting of the balance Fe and unavoidable elements was heated to Ar_
The hot rolled steel strip obtained by hot rolling at a temperature of 3 or more is
A method for producing a high-strength hot-rolled steel sheet with extremely excellent cold workability, characterized by winding at a temperature of 00°C or lower. 9. The method according to any one of claims 5 to 8, which provides extremely excellent cold workability, characterized in that the coiling temperature after hot rolling is 350°C or lower and 100°C or higher. .